The process of making an integrated circuit from a silicon wafer employs photolithography, which includes the step of transferring geometric patterns, called features, to a photosensitive film applied to the surface of the wafer. Some of these features become the various circuit elements (transistors, resistors, capacitors, etc.) of the integrated circuit, and some features are used for alignment and registration.
A stepper, also called a stepping aligner, is an optical system that repeatedly projects a geometric pattern onto the photosensitive film-coated wafer over a sequence of positions on the surface of the wafer. After each projection of the geometric pattern, the wafer is moved, i.e., "stepped," to a new position, and the projection is repeated.
The step of transferring a single geometric pattern at a position on the silicon wafer is called a masking step. During the fabrication of an integrated circuit, several masking steps are performed at each position, each masking step overlaying a different geometric pattern over the previous ones. The pattern that results from a sequence of masking steps is shown in FIG. 1, where the progression from a single mask pattern to a complex overlap of many geometric patterns is indicated by the arrow 18.
Before each masking step, the pattern to be transferred to the wafer must be precisely sized and aligned with respect to the previous patterns. To accomplish this alignment, the surface of the wafer is visually inspected, typically using an optical device. However, each masking step changes the visual appearance of the wafer's surface. These changes in appearance commonly include distortion and degradation of fiducial marks or other reference features used to achieve alignment visually. Consequently, it is difficult to achieve accurate and repeatable visual alignment over a sequence of masking steps.
Further, known alignment techniques require that the reference features be clearly distinguishable with respect to background features and random noise. However, each photolithographic step can change the appearance of the reference features on the wafer to an extent that, after a sequence of masking steps, the reference feature becomes indistinguishable from background image noise. Moreover, a masking step can introduce a change in image polarity, which introduces further difficulty.
A known method for locating a reference feature is to perform a two-dimensional search using, for example, the Search Tool, as sold by Cognex Corporation, Natick, Mass. However, this method is impractical whenever the reference features of the object under inspection are degraded by one or more processing steps. Moreover, since each processing step has an unpredictable effect on the appearance of the object's features, is difficult to train a search model for use with more than one masking step. In addition, rotation may be introduced between masking steps, which a two-dimensional search tool typically cannot accommodate.